A great way to update a room is to add a tactile or visual interest by adding a patterned rug. Our favorite is our Handwoven Rustic Rug which adds texture with the hand sewn cotton and wool. Along with its staggered design makes it a unique piece in any room.
In regards to size, I don’t know if there is a perfect size. I think a lot of it depends on personal preference. If you are unsure of what would look right, just make sure that there is an even border of flooring around all four sides, or at least that borders of flooring opposite each other are even. Don’t get a rug so large that it touches the baseboard around the room.
If you have a large room that doesn’t have a definable space, I suggest putting the front legs of furniture pieces on the rug and the back legs off. However, more importantly, you might want to make sure the rug is placed in a balanced fashion in the room.